1. Field of the Invention
The embodiments disclosed herein relate generally to machine drive systems and more particularly to damping of torsional vibrations in such systems.
2. Description of the Related Art
Prime movers (e.g., gas turbines or electrical motors) are typically connected to their respective loads (e.g., electric generators or compressors) by a mechanical shaft, which may exhibit resonance at one or more critical frequencies excited by the power drive and/or by the load. If a torque component at a critical frequency is generated, then the shaft will experience torque oscillations, which can lead to an increase in fatigue and reduction in the life of the shaft. In extreme cases, if such modes are excited to the point of exceeding the design capacity of any component in the train (especially a coupling that that may be considered the weakest component in the system), severe damage to the system may result due to torsional fatigue, leading to down time and expensive repairs. Moreover, for turbo rotor trains with gears, high radial and lateral vibrations in the gearbox may be observed due to the fact that torsional and lateral motions of the gears may be coupled to one another in the presence of large oscillatory angular motion of the train.
Torsional vibrations are oscillatory angular shaft motions superimposed on the steady-state shaft speed so as to twist various sections of the shaft as well as machine couplings. As already noted, high torsional vibrations may be destructive and result in high lateral vibration at the gearbox. Severe torsional vibrations may be present during the operation of a system with the only indication of a problem being gear noise or coupling wear. Some typical effects of uncontrolled torsional vibration may be a failed coupling, a broken shaft, a worn gear, a fractured gear tooth, and more.
Excitation of torsional natural frequencies may come from many sources. From the electrical drive, the source of the torque components can be voltage harmonics present due to the nature of output modulators in voltage converters or due to the presence of other current or voltage disturbances within the electrical network in connection with the machine. Another main cause for torsional issues in electrical motor driven shaft assemblies are air-gap torque harmonics generated by the electrical drive system.
As those of ordinary skill in the applicable arts understand, the torsional characteristics of these systems vary as a function of the stiffness and inertia of the train. While some torsional properties of the system may be changed, generally the system inertia cannot be easily modified as needed, particularly if torsional issues develop when the project development is completed (i.e., during string tests or during the commissioning phase). Conventional turbo machinery drive trains driven or assisted by an electric motor that include a Variable Frequency Driver (VFD) require specific considerations in the design stage over conventional constant speed equipment. Furthermore, variable-frequency drives are notorious sources of torsional vibration problems due to the torque ripple generated on the motor shaft. One way to reduce these torsional vibrations is to carefully design the entire compressor train from a torsional standpoint with the proper selection of couplings, gearbox and rotors in order to prevent torsional and/or torsio-flexural issues. Nevertheless, optimization of control-loop parameters for train torsional behavior has not been observed as a successful option to address the noted torsional issues.
It would therefore be desirable to develop turbo machinery drive trains driven or assisted by an electric motor that include an alternative way to reduce or smooth the harmful effect of alternating torsional stress acting directly on the source of the excitation with no or minimum impact on the hardware of the system.